The duration of fertility in 995 egg-type hens which were the issue of 36 sires and 176 dams was measured at 33 and 44 weeks of age. D u rin g each period, the hens were intravaginally inseminated (125 x 10® sperm atozoa) on the first and second days and the eggs laid on days 3 to 22 incubated. Fertility was determined by candling after 7 days of incubation. Several traits were considered : the number of days after A . I. d u rin g which all the eggs laid by the hen were fertile ("efficient d u ra tio n "), the number of days between A I and the last fertile egg ("maximum duration "), the number of fertile eggs laid d uring the 20-day period, the total number of chicks hatched from the eggs collected during this period. The heritabilities of the characteristics of duration of fertility varied from 0.12 to 0.31 for the first period and from 0.10 to 0.17 for the second. For a given variable, genetic correlations among the 2 periods were more than 0.79 with the exception of efficient duration which seems dependent on laying rate. Fo r a given period all genetic correlations among these variables (with the exception of efficient duration) were above 0.88. Duration of fertility was positively correlated with laying intensity. The number of chicks hatched appears to be the best criterium for selection. B o x-C o x transformation was used but did not completely achieve normality.